The preparation of di- and polyamines of the diphenylmethane series (MDA) from aniline and formaldehyde using acidic catalysts is generally known. In the context of the present invention, di- and polyamines of the diphenylmethane series are understood as meaning amines and mixtures of amines of the following type:

Here, n is a natural number ≧2. Hereinbelow, the compounds of this type in which n=2 are referred to as diamines of the diphenylmethane series or diaminodiphenylmethanes (subsequently MMDA). Compounds of this type in which n is >2 are referred to within the context of this invention as polyamines of the diphenylmethane series or polyphenylenepolymethylenepolyamines (subsequently PMDA). Mixtures of both types are referred to as di- and polyamines of the diphenylmethane series (subsequently MDA). The corresponding isocyanate, which can be derived formally by replacing all NH2 groups by NCO groups from the compounds of formula (I) are accordingly referred to as diisocyanates of the diphenylmethane series (subsequently MMDI), polyisocyanates of the diphenylmethane series or polyphenylenepolymethylenepolyisocyanates (subsequently PMDI) or di- and polyisocyanates of the diphenylmethane series (subsequently MDI). Here, both in the case of the amine and also in the case of the isocyanate, the polymer (n>2) is generally always present in the mixture with the dimer (n=2), meaning that in practice only two compound types are relevant, the pure dimers (MMDA or MMDI) and the mixture of dimers and polymers (MDA or MDI).
Industrially, the di- and polyamine mixtures are converted predominantly by phosgenation to the corresponding di- and polyisocyanates of the diphenylmethane series. The continuous or partly discontinuous preparation of MDA is disclosed, e.g., in U.S. Pat. No. 5,286,760, EP-A-451442 and WO-A-99/40059. The preparation of MDA can take place by converting aniline and formaldehyde directly in the presence of an acidic catalyst or firstly, in the absence of an acidic catalyst, firstly converting aniline and formaldehyde to the so-called aminal, which is then rearranged in a subsequent process step under acid catalysis to the di- and polyisocyanates of the diphenylmethane series (so-called aminal process). The present invention deals with the aminal process. In both processes, the acid is neutralized after the rearrangement has taken place.
Commercially available formaldehyde generally comprises methanol, either as a result of production and/or intentionally added in order to increase the stability of the formaldehyde. In this connection, formaldehyde grades with (i) low contents of methanol (for customary industrial applications), (ii) medium contents of methanol (for in particular pharmaceutical applications) and (iii) high contents of methanol (for special applications) are available (cf. e.g. “J. Frederic Walker, Formaldehyde, Chapter 4, Commercial Formaldehyde Solutions, Third Edition, Robert E. Krieger Publishing Company, Huntington, N.Y., 1975”). According to the last-mentioned literature reference, the customary methanol content in formaldehyde solutions in case (i) is 0.3 to 1.5%, in case (ii) is 6.0 to 15% and in case (iii) is 32.5 to 43%. The specified limits and differences between the fields of application naturally do not apply in the strict sense. Thus, e.g. the publication “David F. Gould, Phenolic Resins, Reinhold Publishing Corporation, New York, London, 1959” discloses the use of a formaldehyde with an average methanol content (7 to 15%) in the preparation of phenol-formaldehyde resins. However, a technical-grade formaldehyde solution with a low methanol content (at most 2% by mass, generally 1 to 2% by mass) is usually used for the MDA preparation.
The work-up of the acidic reaction mixture obtained in the preparation is triggered according to the prior art by neutralization with a base. According to the prior art, the neutralization usually takes place at temperatures of for example 90° C. to 100° C. without the addition of further substances (H. J. Twitchett, Chem. Soc. Rev. 3(2), p. 223 (1974)). However, it can also take place at a different temperature level in order e.g. to increase the rate of the degradation of troublesome by-products. Hydroxides of the alkali metal and alkaline earth metal elements are suitable as bases. Preferably, aqueous NaOH is used.
After the neutralization, the organic phase is separated from the aqueous phase in a separation container. The organic phase comprising crude MDA which remains after the aqueous phase has been separated off is subjected to further work-up steps such as e.g. a washing with water (base washing) in order to wash residual salts from the crude MDA. Finally, the crude MDA purified in this way is freed from excess aniline, water and other substances present in the mixture (e.g. further solvents) by suitable methods such as e.g. distillation, extraction or crystallization. The work-up customary according to the prior art is disclosed for example in EP 1652 835 A1, page 3, line 58 to page 4, line 13 and EP 2 103 595 A1, page 7, lines 21 to 37.
EP 1 616 890 A1 discloses a process in which aniline and formaldehyde are firstly converted in the absence of the acidic catalyst to aminal, and the aminal obtained in this way is then admixed with an acidic catalyst and converted further at temperatures of 20° C. to 100° C. and at water contents of the acidic reaction mixture obtained in this way of 0 to 20 percent by weight. In particular, after the condensation of formaldehyde and aniline to give the aminal, firstly the water is at least partly removed from the aminal, with a water content of 0 to 5 percent by weight being established in the aminal before the aminal is admixed with acidic catalyst. In this way, it is possible to prepare MDA with a degree of protonation of <15%, preferably 4 to 14%, particularly preferably 5 to 13%. The degree of protonation here for monobasic acidic catalysts (such as hydrochloric acid) is the term used to refer to the molar ratio of the amount of acidic catalyst used and the molar amount of amine functions present in the reaction mixture.
EP 1 813 598 A1 teaches (see in particular paragraphs [0037] to [0042]) that the water produced in the aminal reaction (water of reaction) and the water originating from the formalin is partly or completely combined with other waste water streams of the process and is further treated to remove organic constituents, such as e.g. aniline and MDA, such as e.g. a combination of extraction and distillation. The whereabouts of the feed material formalin is not described. Furthermore, EP 1 813 598 A1 (see paragraph [0043]) teaches that during the distillative aniline removal from the extracted waste water, the vapors can be condensed in multiple stages and, in so doing, a fraction comprising methanol and further low-boilers in high concentrations can advantageously be produced. Firstly, therefore, the methanol level in the process stages is advantageously reduced, and secondly this fraction can advantageously be used as a fuel substitute.
The quality of a process for the preparation of MDA is on the one hand defined by the content in the product of undesired byproducts of the reaction. On the other hand, the quality of a continuous process is defined by the fact that the overall process from start-up, normal production to shutdown of the process can be operated without technical production loss or problems which require intervention in the process, and that there are no resulting losses of feed materials, intermediate products or end product. Such problems can arise e.g. upon adjusting the water content in the aminal by phase separation of organic phase and aqueous phase. Problems of this type can be e.g. that it results in delays during phase separation, or that the phase separation is incomplete, or that a third phase (mulm or mulm layer) is formed. This third phase is a stable, sometimes voluminous interim phase which occurs between the aqueous phase and the organic phase and hinders the phase separation and, in extreme cases, even prevents it entirely. In the most unfavorable case for operational progress, the phase separation container or containers affected have to be completely emptied and cleaned. The content of the phase separation container or containers then has to be worked up, which is complex, or be disposed of, which is associated with considerable costs. In some circumstances, this can also lead to the continuous production having to be interrupted. If the formation of a mulm layer cannot be completely avoided, then this will ultimately end up in one of the two phases. If the mulm layer ends up in the organic phase, then, in the case of phase separation after the aminal reaction, this is less serious than if it ends up in the aqueous phase. This is because in the last-mentioned case, larger amounts of dispersely dissolved organic materials then end up in the aminal water with the mulm layer. Said losses can then arise during the disposal or further use of the aminal water.
EP 2 103 595 A1 deals with a procedure for the preparation of di- and polyamines of the diphenylmethane series in which aniline and formaldehyde are converted in the presence of an acidic catalyst. In connection with the phase separation after the neutralization of the crude product, it is disclosed that this phase separation can be assisted by adding water and/or aniline. Preferably, the reaction mixture diluted by adding water and/or aniline is separated into an organic phase and aqueous phase in separating flasks with plate sections, assisting the coalescence of the two phases, as internals (paragraphs [0043] and [0044]). Apart from the fact that the use of mechanical separation aids results in an additional expenditure on apparatus, completely satisfactory results cannot be achieved with plate sections if particularly high requirements are placed on the quality of a phase separation. This applies accordingly for the separation of the two phases after the aminal reaction for a process procedure for the preparation of di- and polyamines of the diphenylmethane series in which aniline and formaldehyde are reacted firstly in the absence of an acidic catalyst and the aminal formed is only rearranged in a subsequent process step with acid catalysis.
It would therefore be desirable to have available processing measures in order to be able to overcome these problems.
Although the described processes of the prior art succeed in preparing MDA with a high yield, no auxiliaries are described which, without additional expenditure on apparatus, could improve the separation of the organic phase from the aqueous aminal phase with the desirable efficacy in order to minimize the losses of feed materials and intermediate products in the reaction process and to ensure a seamless technical progress of the production process.
There was thus a need for a process for the preparation of di- and polyamines of the diphenylmethane series in which it is possible, by means of simple measures, to conduct an improved phase separation between organic phase and aqueous phase in the aminal stage. This would improve the cost-effectiveness of existing MDA processes.